Conventionally known optical dew point meters are designed such that a chilling means such as a multi-stage Peltier device and a Freon freezer or a liquid nitrogen or helium refrigerator is combined with a heater means to thereby permit the varying of the temperature of a reflector mirror to the subliming point of CO.sub.2 and below. When a sample gas is brought into contact with said reflector mirror, and followed by the projection of condensed rays of light or laser light onto dew or frost formed on the reflector mirror, the change in the intensity of the resulting scattered light and/or reflected light caused by varying of the temperature of the reflector mirror can be detected and it is possible to know the dew point or frost point of the sample gas and to thereby determine its water content. A major problem of these optical dew point meters is that the reflector mirror is fouled by fine particles, oil components, etc. during continued measurements. In particular, the precision, sensitivity and detection limit for the measurement of dew point or frost point are greatly influenced if the reflector mirror is fouled. Many proposals have been made on the basis of many years of studies conducted on the method of cleaning a fouled reflector mirror but it has been difficult to retain satisfactory precision after cleaning.
For example, U.S. Pat. No. 3,623,356 describes a method in which the temperature at the surface of a reflector mirror is elevated periodically to the dew point or higher, whereby all dew or frost is evaporated from the reflector mirror and, thereafter, the degree of fouling of the reflector mirror is checked before starting the cleaning operation. Japanese Patent Public Disclosure Nos. 154652/1981 and 151347/1981 proposed methods for continuous measurement. In those methods, the value for a reflector mirror is corrected on the basis of the degree of fouling of a reference plane that is close to tile reflector mirror and which is maintained at a higher constant temperature and if the degree of fouling of the reference plane is found to have reached a certain value, the reflector mirror is cleaned by heating it above the dew point. However, these methods are intended to achieve a certain recovery of the reflectance of the mirror, not to a level completely equal to the reflectance as measured when making quality inspection before shipment from the factory. In order to achieve complete recovery to the initial reflectance, the reflector mirror has to be returned to the factory for readjustment at considerable cost.
A further problem with the optical dew point meter is that it is airtight in the interior. If it is disassembled for cleaning, it must be reassembled and this is a cumbersome operation. In addition, the reassembled instrument must be tested for airtightness. Because of the need to perform these operations, the cleaning of the dew point meter necessitates its shutdown for a period of from at least one week to one month.
Instead of disassembling it, one may contact the reflector mirror with a gas containing the vapor of a cleaning organic solvent. This method is also capable of cleaning the reflector mirror but it is not easy to prepare a suitable organic solvent of high quality. In addition, the organic solvent used will build up in the airtight chamber and it takes considerable time to remove it by evaporation. Of course, cleaning with Freon is also effective but its use should be restricted to preserve the atmospheric environment and from safety and hygenic view-point.